1. Field of the Invention
The present invention relates to a method of manufacturing a tandem-type thin-film solar cell and more particularly to the method able to improve flexibility of the manufacturing process and production efficiency without deteriorating performance of the solar cell.
2. Description of the Background Art
In recent years, the variety of thin-film solar cells has been increasing and not only the conventional amorphous thin-film solar cells but also crystalline thin-film solar cells have been developed. Moreover, tandem-type (including hybrid type) thin-film solar cells which each have stacked amorphous and/or crystalline photoelectric conversion units are going to be put into practical use.
A semiconductor thin-film solar cell generally includes a first electrode, at least one semiconductor thin-film photoelectric conversion unit and a second electrode that are stacked successively on a substrate having an insulating property at least on its surface. One photoelectric conversion unit includes an i-type layer sandwiched between a p-type layer and an n-type layer.
The i-type layer occupying a major part of the overall thickness of the photoelectric conversion unit is substantially an intrinsic semiconductor layer mainly in which photoelectric conversion process occurs. Accordingly, the i-type photoelectric conversion layer is preferably thicker in terms of light absorption, while the thickness thereof increased more than necessary increases the cost and time required for depositing the i-type layer.
On the other hand, the p-type and n-type layers serve to cause a diffusion potential in the photoelectric conversion unit. The magnitude of the diffusion potential determines a value of an open circuit voltage which is one of important characteristics of the thin-film solar cell. However, these p-type and n-type layers are inactive layers that do not directly contribute to the photoelectric conversion. Therefore, light absorbed by impurities in the p-type and n-type layers results in optical loss which makes no contribution to power generation. Then, the p-type and n-type layers preferably have respective thicknesses as thin as possible in the range which can cause a sufficient diffusion potential.
Accordingly, a photoelectric conversion unit or thin-film solar cell is referred to, regardless of whether p-type and n-type conductive layers thereof are amorphous or crystalline, as amorphous unit or amorphous thin-film solar cell if the i-type photoelectric conversion layer occupying the major part thereof is amorphous, and as crystalline unit or crystalline thin-film solar cell if the i-type layer is crystalline.
Conversion efficiency of a thin-film solar cell can be enhanced by stacking at least two photoelectric conversion units thereby making the cell tandem-type. Specifically, a front unit including a photoelectric conversion layer having a wide band gap is placed on a light-incident side of the thin-film solar cell and a rear unit including a photoelectric conversion layer (of Sixe2x80x94Ge alloy for example) having a narrow band gap is placed behind the front unit. Then, photoelectric conversion for a wide wavelength range of incident light is achieved to enhance the conversion efficiency of the entire solar cell. In particular, a tandem-type thin-film solar cell including both of amorphous and crystalline photoelectric conversion units is sometimes referred to as hybrid thin-film solar cell.
For example, an i-type amorphous silicon is capable of photoelectrically converting light having a wavelength up to approximately 800 nm, while an i-type crystalline silicon is capable of photoelectrically converting light having a still longer wavelength up to 1100 nm. Here, the amorphous silicon photoelectric conversion layer has a large light absorption coefficient and its thickness of 0.3 xcexcm or smaller is enough for light absorption. On the other hand, the crystalline silicon photoelectric conversion layer having a small light absorption coefficient preferably has a thickness of approximately 2 to 3 xcexcm or greater for absorbing sufficient longer-wavelength light. In other words, it is usually desirable for the crystalline photoelectric conversion layer to have a thickness approximately ten times as large as that of the amorphous photoelectric conversion layer.
When the tandem-type thin-film solar cell includes both of the amorphous and crystalline units, the optimum plasma CVD conditions for forming the amorphous unit are different from those for forming the crystalline unit. Then, the amorphous and crystalline units are preferably formed under respective optimum conditions in separate plasma CVD apparatuses each including a vacuum chamber for CVD process. In addition, formation of the crystalline unit requires a longer time compared with that required for formation of the amorphous unit. Then, it may be desirable to rapidly produce the crystalline units through a plurality of manufacturing lines over the amorphous units produced through a single manufacturing line. Moreover, even when the tandem-type thin-film solar cell includes a plurality of crystalline units only, a front unit closer to a light-incident side of the cell and a rear unit arranged behind the front unit are made to have their respective thicknesses and other different characteristics in order to optimize the light absorption efficiency. Then, it is often desirable to form respective units by separate plasma CVD apparatuses.
However, under the situation as described above, when a p-i-n-type amorphous unit including a junction of p-i-n in this order from a transparent substrate is formed and the substrate is then removed from a plasma CVD apparatus temporarily into the atmosphere and further introduced into another plasma CVD apparatus to form a p-i-n crystalline unit thereon, a resultant tandem thin-film solar cell has photoelectric conversion characteristics inferior to that of a tandem thin-film solar cell manufactured by successively forming both units without taking out the substrate into the atmosphere. This fact has actually been experienced by the inventors of the present invention. Specifically, respective photoelectric conversion efficiencies were compared by means of absolute values thereof and the former is inferior to the latter by at least 0.5%.
An object of the present invention in consideration of the fact experienced by the inventors is to provide a method of manufacturing a tandem thin-film solar cell to improve flexibility of a manufacturing process and production efficiency thereof without deteriorating performance of the tandem thin-film solar cell.
According to an aspect of the present invention, a method of manufacturing a tandem thin-film solar cell is provided. The solar cell includes a plurality of photoelectric conversion units stacked on a substrate which each include a p-type layer, an i-type photoelectric conversion layer and an n-type layer formed in this order from a light-incident side of the solar cell. At least a rear unit among the photoelectric conversion units furthest from the light-incident side is a crystalline unit including a crystalline i-type photoelectric conversion layer. The method includes the steps of forming at least one of the units on the substrate by plasma CVD and immediately thereafter forming an i-type boundary layer to a thickness of at most 5 nm by plasma CVD, and thereafter removing the substrate into the atmosphere to expose a surface of the i-type boundary layer to the atmosphere and then forming a crystalline unit on the i-type boundary layer by plasma CVD.
Preferably, when the photoelectric conversion units each include a p-type layer, an i-type photoelectric conversion layer and an n-type layer in this order from the substrate, an additional n-type layer is formed by plasma CVD immediately before the crystalline unit is formed on the i-type boundary layer.
Preferably, an amorphous unit including an amorphous i-type photoelectric conversion layer is formed on the substrate as a firstly formed unit and as a front unit closest to the light-incident side.
Preferably, a source gas containing SiH4 diluted with H2 may be used for the plasma CVD.
According to another aspect also of the present invention, a method of manufacturing a tandem thin-film solar cell is provided. The solar cell includes a plurality of stacked photoelectric conversion units which each include a p-type layer, an i-type photoelectric conversion layer and an n-type layer deposited in this order on a transparent substrate. At least a rear unit among the photoelectric conversion units furthest from the substrate is a crystalline unit including a crystalline i-type photoelectric conversion layer. The method includes the steps of forming at least one of the units on the substrate by plasma CVD and thereafter removing the substrate into the atmosphere to expose, to the atmosphere, an outermost surface of the unit having already been formed, and then exposing the outermost surface to hydrogen plasma and immediately thereafter forming the crystalline unit by plasma CVD without removing the substrate into the atmosphere.
Preferably, at least one of the units is formed on the substrate by plasma CVD, an i-type boundary layer is immediately thereafter formed to a thickness of at most 5 nm by plasma CVD and then the substrate is removed into the atmosphere.
Substantially the same pressure may be used in the same vacuum chamber to expose the outermost surface of the unit previously exposed to the atmosphere to hydrogen plasma and to thereafter deposit the p-type layer of the crystalline unit by plasma CVD.
Preferably, after the exposure to hydrogen plasma and before deposition of the p-type layer of the crystalline unit, an additional n-type microcrystalline intermediate layer is deposited by plasma CVD, and the n-type microcrystalline intermediate layer may be deposited at substantially the same gas pressure as that at which the exposure to hydrogen plasma is performed.
Preferably, an amorphous unit including an amorphous i-type photoelectric conversion layer is formed on the substrate as a firstly formed unit.
Preferably, a source gas containing SiH4 diluted with H2 is used for the plasma CVD.
According to still another aspect also of the present invention, a method of manufacturing a tandem thin-film solar cell is provided. The solar cell includes a plurality of stacked photoelectric conversion units which each include a p-type layer, an i-type photoelectric conversion layer and an n-type layer deposited in this order on a transparent substrate. At least a rear unit among the photoelectric conversion units furthest from the substrate is a crystalline unit including a crystalline i-type photoelectric conversion layer. The method includes the steps of forming the n-type layer included in at least one of the units on the substrate by plasma CVD with a relatively low dopant concentration and thereafter removing the substrate into the atmosphere to expose a surface of the low-concentration n-type layer to the atmosphere, and performing plasma CVD to thereafter form an n-type intermediate layer on the low-concentration n-type layer previously exposed to the atmosphere, the intermediate layer having a higher concentration than that of the low-concentration n-type layer, and immediately thereafter form the crystalline unit.
The low-concentration n-type layer preferably has a dopant concentration less than 1.5xc3x971020 cmxe2x88x923 and the n-type intermediate layer with higher concentration preferably has a dopant concentration more than 5xc3x971020 cmxe2x88x923.
Preferably, the surface of the low-concentration n-type layer exposed to the atmosphere is then exposed to hydrogen plasma and immediately thereafter the n-type intermediate layer with higher concentration is formed by plasma CVD.
Substantially the same gas pressure may be used in the same vacuum chamber to expose the surface to hydrogen plasma, deposit the n-type intermediate layer by plasma CVD and deposit the p-type layer of the crystalline unit on the intermediate layer by plasma CVD.
Preferably, an amorphous unit including an amorphous i-type photoelectric conversion layer is formed on the substrate as a firstly formed unit.
Preferably, a source gas containing SiH4 diluted with H2 may be used for the plasma CVD.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.